内置累加整型计数器。初始值为0
官方源码如下：
class Accumulator {
/// [Accumulator] may be initialized with a specified value, otherwise, it will
/// initialize to zero.
Accumulator([this._value = 0]);
/// The integer stored in this [Accumulator].
int get value => _value;
int _value;
/// Increases the [value] by the `addend`.
void increment(int addend) {
assert(addend >= 0);
_value += addend;
}
}
例子：
final num = Accumulator();
num.increment(10);
if (kDebugMode) {
print(num.value);
}
num.increment(8);
if (kDebugMode) {
print(num.value);
}
输入结果为10、18
